1. Field of the Invention
The present invention relates to a vehicle steering system that includes a transfer ratio control mechanism and a steering power assisting mechanism.
2. Description of the Related Art
There is a vehicle steering system that includes a transfer ratio control mechanism for controlling the transfer ratio of the steering angle of the steering wheel to the steered angle of the steered wheels (i.e. front wheels) together with a steering power assisting mechanism for giving a driver an assisting steering power. The transfer ratio control mechanism is usually equipped with a gear mechanism and a gear driving motor.
However, if the steering wheel of a vehicle that is equipped with the above vehicle steering system is steered at a high steering speed when the vehicle runs at a low speed, the front wheels of the vehicle that are steered may turn at such a high speed that the steering power assisting mechanism can not follow the speed. As a result, the driver may have to add much more power than expected steering power to steer the steering wheel.
In order to solve this problem, JP-A-2000-344120 proposes a steering system in which the transfer ratio control mechanism increases the transfer ratio when the steering speed becomes higher than a preset speed. However, this steering system does not assist a driver very much to steer the vehicle when the vehicle runs at a low speed; and the driver may have to turn the steering wheel more than he desires.